Metallic effect pigments have been used for many years in coatings in order to generate a metallic effect.
Conventional metallic effect pigments consist of platelet-shaped metallic pigments whose effect derives from the directed reflection of incident light from metal particles of planar form which are oriented in parallel in the respective application medium.
Typical fields of application of metallic effect pigments are the coatings industry, especially the automotive industry, the printing industry, and the plastics industry.
The metallic effect is described by certain parameters. These parameters include the brilliance (sparkle and metallic luster), the lightness, and the flop (change in lightness as a function of incident angle and/or viewing angle), and the covering power. In the case of colored metallic coatings, further parameters are the chroma and the color flop (“two-tone”).
The gloss is determined according to the proportion of reflected to scattered light in relation to a standard.
Key factors influencing the metallic effect include the particle morphology and the form factor (ratio of average particle diameter to average particle thickness) of the pigments, the thickness of the particles and also their surface roughness, the particle size, the particle-size distribution, and the orientation of the pigment parallel to the surface of the coating material or plastic.
In relatively large-diameter pigment particles of uniform morphology the reflection is relatively high, this being manifested in high metallic brilliance, improved lightness, and strong flop, whereas, for relatively low-particle-diameter pigments, the scattering fraction is very high, resulting in good covering power.
The covering power is determined above all, however, by the thickness of the metallic pigments. The thinner the metallic pigments, the better their specific covering power, i.e., the covering power per unit weight.
On the part of the printing, coatings, plastics, and cosmetics industries there is great interest in colored metallic pigments, and particularly in metallic pigments with a golden luster. Products resembling gold possess a high esthetic quality and give the materials thus coated, printed or colored a valuable appearance.
In recent times, however, black metallic effects as well have been enjoying increasing popularity. For instance, the color black has presently become the fashion color in the automobile sector. These effects are based on mixtures of black pigments with conventional aluminum pigments. Metallic pigments which are inherently black and also have a high gloss have not so far made a commercial entrance.
Very well established are the pigments known as gold bronze powders, which consist predominantly of copper/zinc alloys and which depending on composition may have different hues from red gold to rich gold (Pigment Handbook, vol. 1, 1973, p. 807 ff., Wiley-Interscience). Gold bronze pigments are produced by atomization of a liquid copper/zinc alloy melt and subsequent milling of the powder formed during atomization. In the course of the milling operation, the alloy particles are deformed to a platelet shape and comminuted. In the art, gold bronze pigments are obtained predominantly by dry milling. In order to avoid instances of cold welding, lubricant such as stearic acid, for example, is added to the atomized powder employed. Irregularities on the surface and edges of the metal platelets have the effect of reducing luster. These conventionally manufactured metallic effect pigments possess not only a pronounced particle-size distribution but also particle thicknesses of well above 100 nm.
For higher-value applications, particularly thin aluminum pigments have been developed, which are produced via PVD techniques.
Metallic pigments produced by PVD techniques have been known for some considerable time. They are notable for extremely high gloss, an enormous covering power, and unique optical properties. Owing to their low thickness of around 30 to 70 nm and their extremely smooth surfaces, they have a tendency, following application, to conform very closely to the substrate. If the substrate itself is very smooth, the result is virtually a mirrorlike appearance.
Of the pure metallic pigments, only aluminum pigments have made a commercial entrance to date. Examples thereof are Metalure® (manufactured by Avery Dennison, sold by ECKART), Decomet® (Schlenk) or Metasheen® (Ciba). Such pigments represent the “silver” hue in its highest embodiment.
High-value colored metallic effects are generally obtained by blending PVD aluminum pigments with dyes and/or color pigments. In this way, for example, it is possible to generate high-value gold hues by blending the PVD aluminum pigments with yellow dyes or color pigments. Such blends, however, have disadvantages: for instance, these blends cannot be applied in particular to absorbent substrates, owing to the separation there of metallic pigment from dye. In applications requiring high light fastnesses, these systems often fail because of the deficient light fastness of the colored pigment or of the dye.
Pigments based on metallic layers and produced via PVD techniques are described in more detail in U.S. Pat. No. 2,839,378.
Described therein is the manufacture of mirrorlike pigments with extremely thin layer thicknesses, which are applied by vapor deposition to a substrate provided with a “release layer”. After the metal layers have been applied and the film detached, the pigments are comminuted to the desired particle size by means of mechanical action.
The application of pigments manufactured in this way in coating formulations is described in detail in U.S. Pat. No. 2,941,894. That patent emphasizes the high reflectivities, the low level of pigmentation, and the high hiding power or covering power of the pigments.
The operation of producing metallic pigments by means of vapor deposition processes with a thickness of 35 to 45 nm is described with greater precision in U.S. Pat. No. 4,321,087, and involves the application of a release coat, the metallizing operation, the detachment operation in a solvent bath, the concentrating of the particles, and their ultrasonic comminution to the desired pigment size.
These one-layer metallic pigments have a limited diversity of hue. There is a need for new color effects with optically high-grade metallic pigments.
WO2004/026971 and WO2004/026972 relate to one-layer, high-luster, golden metallic effect pigments which are composed of a copper-based alloy and other metallic alloying constituents and are manufactured by detachment and comminution of metal films deposited under vacuum. The disadvantages of such pigments are the limited diversity of hue. The manufacture of pigments using Cu or Zn as heavy metal leads to pigments which have a high density and, in association therewith, a relatively low covering power, and also leads to sedimentation problems in certain formulations. Another disadvantage lies in the high sensibility of these alloy pigments to corrosion.
Multilayer effect pigments manufactured by PVD techniques (Physical Vapor Deposition) have also been known for a long time. They were first described in U.S. Pat. No. 3,438,796. Claimed therein are five-layer interference pigments having a central, reflecting aluminum layer, flanked on either side by an SiO2 layer with a thickness of 100 to 600 nm and, lastly, by semitransparent absorber layers comprising aluminum. The central aluminum layer is to have a reflecting effect, i.e., layer thicknesses of more than 60 nm are needed for this purpose. The external aluminum absorber layers, in contrast, must possess layer thicknesses of below 40 nm in order to have semitransparency properties. In that patent, furthermore, an interference pigment having a three-layer construction was described, in which a central SiO2 layer is flanked by two thin, semitransparent aluminum layers.
U.S. Pat. No. 5,571,624 claims a paint which comprises multicolor interference pigments. These pigments possess a central metallic reflecting layer, flanked on either side by layer stacks composed in turn of a dielectric and a semiopaque metal layer, the dielectric layer facing the reflector core. Here again, in order to be truly opaque, the central metallic reflecting layer is required to have a minimum thickness of 35 to 40 nm. The dielectric layers ought to possess at least an optical layer thickness of two quarters of a selected wavelength of 400 nm. For an SiO2 layer, for example, with a refractive index of 1.55, this corresponds to a geometric minimum layer thickness of 310 nm.
Golden metallic pigments of high quality are disclosed in DE 10 2004 063433 A1. Described therein are multilayer PVD pigments which have a central metal layer so thin that it no longer has an opaquely reflecting effect. On either side this layer is coated with dielectric layers. The manufacture of these kind of pigments is inevitably expensive, since producing a pigment layer on the detachment foil requires the latter to be coated three or five times. The production of the absorbing central layer cannot easily be reproduced under production conditions.
Similar pigments are disclosed in WO 2004/052999 A2. They have the same disadvantages.
These multilayer effect pigments all have the disadvantage that the dielectric layers, in comparison to metal layers, can be applied by vapor deposition only at very slow rates. Consequently, multilayer effect pigments produced by vaporization techniques, in which dielectrics are vaporized or vapor-deposited, can be produced only very cost-intensively. Furthermore, a foil has to be vapor-coated a plurality of times in order for the multilayer structure to be realizable, and this pushes the manufacturing costs up further.
EP 1 522 606 A1 describes the production of a foil with black aluminum oxide. Neither effect pigments nor multilayer structures are disclosed therein. The films disclosed there have no notable metallic effect with luster and flop.
U.S. Pat. No. 4,430,366 describes the production of films which comprise a mixture of metal and metal oxide. Here again, no effect pigments are mentioned. The films possess an inhomogeneous composition with a gradient of metal and metal oxide over the layer thickness, the metal concentration gradient and the metal oxide gradient being contrary to one another.